The attack occurred on Wednesday at Alaagba village in Ogun state after the Chinese workers were ambushed, state police spokesman Abimbola Oyeyemi told AFP.

"Four Chinese nationals working at the construction site of the railway around Alaagba area were abducted by some gunmen who also shot dead their police escort," he said.

He said police launched a manhunt for the kidnappers to rescue the hostages.

President Muhammadu Buhari commissioned the $2.5 billion, 157-kilometre Lagos-Ibadan standard gauge rail line last week.

Chinese firms are working in Nigeria on multi-billion-dollar infrastructure projects that include mining, railways, airports and roads.

Kidnapping for ransom used to be concentrated in Nigeria's oil-producing south, but the crime has lately spread to other parts of the country.

The victims are usually released after a ransom is paid although police rarely confirm if money changes hands.

In April, two Chinese workers were abducted and their private security guards shot at a mining site in southwest Osun state. They were released four days later.